1. Field of the Invention
The present invention relates to a head IC, read circuit and medium storage device for adjusting an output level of a read signal from a head, and more particularly to a head IC, read circuit and medium storage device for adjusting the read signals from the head, of which output levels are different, to a predetermined level.
2. Description of the Related Art
In a medium storage device, such as a magnetic disk device, a head reads data from a medium. Recently as track densities increase, heads, of which sensitivity to detect magnetic flux of a medium is high, are being provided, where an MR (Magnetic Resistance) element is used for a read head. On the other hand, the use of a TuMR (or TMR) element utilizing the tunnel effect is under consideration as an element having even higher detection sensitivity.
However the output level of such read elements are not constant, and the characteristic change of the element caused by temperature change and fluctuation of floating amount also changes the output level. Therefore a circuit to adjust the output level of the read element before demodulating data is required.
FIG. 11 is a diagram depicting a configuration of a conventional medium storage device, and FIG. 12 is a block diagram depicting a conventional read circuit. As FIG. 11 shows, magnetic heads 202a and 202b, for reading data on a medium (magnetic disk) 210, are installed at the tip of an actuator 200.
The actuator 200 rotates on a rotation axis 204, and positions the magnetic heads 202a and 202b to a desired track on the magnetic disk 210. The magnetic heads 202a and 202b are connected to a head IC 220 installed in an actuator 200, and the head IC 220 is connected to a control circuit 230 outside the actuator 200.
The head IC 220 is installed between the magnetic heads 202a and 202b and the control circuit 230, so that the signal level is adjusted, since a distance from the magnetic heads 202a and 202b to the control circuit 230 is long, so the signal level from the magnetic heads 202a and 202b to the magnetic head may change, or noise may enter. This head IC 220 also integrates many signal lines between the magnetic heads and the control circuit, so as to simplify wiring. For example, the control circuit 230 and the head IC 220 are connected via a serial interface, and the head IC 220 and the magnetic heads 202a and 202b are connected by an individual signal line respectively.
FIG. 12 is a detailed diagram of the read system circuit in FIG. 11, where a variable gain amplifier (preamplifier) 220-1 is installed in the head IC 220 connected to a read element 202-1 of the magnetic head 202a (or 202b), so that the output of the read element 202 is amplified with a gain being set. A gain for each head is set for the variable gain amplifier 220-1.
In the control circuit 230 connected to the head IC 220, a read channel 230-1 is installed, and an AGC (Automatic Gain Control) amplifier 230-2 installed in the read channel 230-1 receives the output of the variable gain amplifier 220-1 of the head IC 220.
The AGC amplifier 230-2 is comprised of a differential amplifier 230-3 and an AGC circuit 230-4. The AGC circuit 230-4 compares an output value of the differential amplifier 230-3 and a reference output value, feeds back the comparison result, and adjusts a gain of the differential amplifier 230-3, thereby adjusting an output level of the differential amplifier 230-3 to a reference level (see Japanese Patent Application Laid-Open No. H10-021647, and Japanese Patent Application Laid-Open No. S64-062806).
This AGC circuit 230-4 sets high-speed feedback coefficients (frequency, gain change amount) for following up a change in one sector of a track of the magnetic disk 210, and equalizes the signal level in one sector.
In prior art, the fluctuation of the output level due to the characteristics of the element itself and the fluctuation of such environmental conditions as the floating amount are adjusted by the AGC amplifier of the read channel.
However along with the recent improvements in recording density, the use of a TuMR element, of which detection sensitivity is higher than the read element, is desired. The detection sensitivity of this read element having high detection sensitivity is about 10 times that of an MR element, but the dispersion of the signal output level is also high accordingly. Also the fluctuation of the signal level, due to temperature change and the fluctuation of the floating amount, is also high since the detection sensitivity is high.
If such a major fluctuation of a signal level occurs, it may be difficult to adjust this fluctuation by the AGC amplifier in the read channel.
Also because of improvements in recording density, circuit frequency is becoming higher, high speeds are demanded for the AGC amplifier of the read channel, and the input dynamic range of the AGC is becoming narrower. This makes it difficult to adjust level fluctuation by a change of the AGC amplifier of the read channel.
In this way, when a read element with high sensitivity is used or when an operation frequency of a circuit is increased to improve recording density, using the AGC of the read channel for adjusting the level fluctuation has limitations.